Water is an important natural resource that is essential to life. Approximately 71 percent of the surface of the Earth is occupied by water. However, only 2.5 percent of the water found on Earth is considered fresh water (i.e., is not salt water or brackish water; both of which are unfit for human consumption). Furthermore, 98.8 percent of this fresh water is contained in ice and ground water. Less than 0.3 percent of all fresh water may be found in surface water bodies such as lakes and rivers. Contaminated waters are detrimental to the environment and public health. Consequently, regulations governing the treatment and monitoring of contaminated water exists in developed countries such as the United States of America. Similar standards for the treatment and monitoring of contaminated waters are emerging in developing countries across the globe.
Although dependent on the source and nature of the water pollutant(s), the treatment of a contaminated water involves a series of processing steps that are arranged to meet a specific treatment objective, or treatment objectives, with maximum efficiency and minimum total life-cycle cost (i.e., the combination of capital and operating costs for a pre-determined operating life typically defined as 20 years). Contaminated waters include, but are not limited to, reclaimed water, potable water, storm water, industrial wastewater, and municipal wastewater. The latter, municipal wastewater for example, contains both particulate and dissolved organic pollutants and nutrients—primarily the macronutrients nitrogen and phosphorus. Processing municipal wastewater with a centralized wastewater treatment plant involves four main process components: preliminary, primary, secondary, and tertiary treatment. The first process component serves to remove large, non-biodegradable particulate matter and is known as preliminary treatment (e.g., screening and grit removal). The second process component serves to remove readily settleable organic and inorganic particulate matter and is known as primary treatment. Primary treatment is accomplished with sedimentation basins, or primary clarifiers, and dissolved air flotation units.
The third process component is known as secondary treatment and incorporates a biological wastewater treatment process. A secondary treatment process typically includes a biological reactor, or bioreactor, and liquid-solid separation unit process. Together, the bioreactor and liquid-solid separation unit processes (e.g., sedimentation basin, dissolved air flotation, or membranes) remove biodegradable organic matter (dissolved and particulate) and suspended solids. When designed to do so, the bioreactor and liquid-solid separation unit processes are also capable of nutrient removal (e.g., nitrogen, phosphorus, or nitrogen and phosphorus). The bioreactor maintains specific environmental conditions required to develop and maintain a bacterial population that is capable of biochemically oxidizing (e.g., organic pollutants quantified as five-day biochemical oxygen demand, BOD5; ammonia-nitrogen, NH3—N) or reducing (e.g., nitrite-nitrogen, NO2—N; nitrate-nitrogen, NO3—N) pollutants in the contaminated water stream depending on the treatment objective. The liquid-solid separation unit process separates bacteria and particulate matter remaining in the effluent stream of the bioreactor from the treated water. Bacteria may exist in biological flocs (i.e., suspended growth) or in a biofilm. The fourth process component is tertiary treatment. A variety of tertiary treatment processes exist depending on the treatment objective and may include chemically enhanced tertiary clarification (for phosphorus removal), granular media filtration (e.g., with sand filters), or advanced oxidation processes. Disinfection of the wastewater treatment plant effluent prior to discharging the effluent stream may be accomplished with chlorine or ultraviolet light, to name a couple disinfection alternatives and is typically included in the definition of tertiary treatment.
The biochemical transformation of dissolved organic compounds, that is, the third process component that is typically characteristic of centralized municipal wastewater treatment plants, is most commonly carried out using a suspended growth process (i.e., a variation of the activated sludge process). Suspended growth processes include microorganisms (as bacteria) that biochemically transform pollutants—typically organic matter and the nutrients nitrogen and phosphorus in the contaminated water stream—into biomass and other reaction by-products.
Suspended growth processes can be modified such that there is also biofilm. In such a case, the process is compartmentalized and the respective bacterial forms are referred to as the suspended growth compartment and biofilm compartment. Biofilms are a thin, usually resistant, layer of microorganisms (as bacteria) that form on and coat various surfaces. The surfaces upon which biofilms grow are known as substratum. Biofilms are typically used for the oxidation of readily biodegradable organic matter (or organic matter that can easily diffuse into the biofilm) and/or the oxidation or reduction of nitrogenous compounds from contaminated water, either alone or combined with suspended growth in a single bioreactor. When used in conjunction with a suspended growth compartment, the biofilm area is established to support the growth of slow-growing bacteria that would otherwise not exist in the suspended growth compartment in a significant quantity at the solids residence time characteristic of the suspended growth compartment. The use of suspended growth and biofilm compartments together allows a process to meet the treatment objective(s) that would otherwise require additional bioreactor volume and secondary clarifier area. Thereby, the capital cost due to construction and land that is required to add process tanks and process mechanical equipment is avoided which typically results in substantial cost savings.
In other cases, provision and maintenance of a biofilm results in a more constant microorganism population which maximizes contaminated water treatment efficiency and consistency. Bioreactors that use only a biofilm compartment include the trickling filter (TF), rotating biological contactor (RBC), biologically active filter (BAF), moving bed biofilm reactor (MBBR), fluidized bed biofilm reactor (FBBR), granular sludge reactor (GSR), and membrane biofilm reactor (MBfR). Systems that make use of both suspended growth and biofilm compartments are commonly referred to as integrated fixed-film activated sludge (IFAS) processes. The substratum for biofilms used to treat contaminated water includes powdered natural lingo-cellulosic materials, sand (particulate biofilms), non-biodegradable bacterial materials (i.e., granular sludge), and man-made materials such as polystyrene and high-density polyethylene.
Skillicorn, U.S. Pat. No. 7,481,934 describes the use of kenaf fibers (a powdered natural lingo-cellulosic material) that act both as biodegradable adsorbent and substratum for biofilm growth when combined with suspended growth in an activated sludge process for contaminated water treatment. The biofilm is allowed to settle in the liquid-solid separation unit (assumed to be a sedimentation basin), along with the suspended growth, receiving bioreactor effluent. In some cases, the liquid-solid separation process is aided by chemical flocculation (i.e., chemically enhanced clarification), and the biofilm/suspended growth mixture is separated from the treated water. In other cases, some of the biofilm/suspended growth mixture is returned to the bioreactor influent (i.e., via a return activated sludge stream) as an inoculant for an additional treatment cycle.
Brown, U.S. Patent Pub. Nos. 2013/0233,792 and 2013/0233,794 describes the use of lignocellulosic fibers, such as kenaf fibers, to form biofilms containing both aerobic and anaerobic bacteria. For example, biofilms containing both aerobic and anaerobic zones may support the development of ammonia oxidizing bacteria (AOB) in the aerobic zones and anaerobic ammonia oxidizing bacteria (Anammox) in the anaerobic zone; therefore, the biofilm may convert ammonia-nitrogen to nitrogen gas in a single bioreactor without recirculation and the provision of an external carbon source. The biofilm is allowed to settle with suspended growth in the liquid-solid separation unit process. No separation of the biofilm from other solids is carried out.
Veolia Water Solutions & Technologies sells ANOXKALDNES® MBBR and hybrid biofilm-activated sludge (HYBAS®) processes. Both of these processes include free-moving plastic biofilm carriers that are retained in a specific bioreactor, or bioreactor zone, with stainless steel screens constructed of wedge wire or perforated plates. The screens are included in the process package along with stainless steel, medium-bubble diffusers in aerobic zones and/or curved-blade mixers in anoxic zones. The free-moving plastic biofilm carriers typically range in size from 10 to 50-mm in diameter and resemble a honeycomb. The free-moving plastic biofilm carriers are permanently retained within the bioreactor.